1. Field of the Invention
The present invention relates to an acceleration sensor and can be applied to an acceleration sensor having a sealing upper substrate with an opening facing on an electrode portion.
2. Description of the Background Art
An acceleration sensor has a structure in which a sealing upper substrate and a sealing lower substrate (hereinafter, referred to simply as “upper substrate” and “lower substrate”, respectively) seal a sensor portion and the like. Further, the acceleration sensor has an electrode portion provided between the upper substrate and the lower substrate, which is connected to the sensor portion, a fixed electrode and the like.
In order to connect the electrode portion to an external wire, conventionally, an opening is provided in the upper substrate.
Specifically, the opening which faces on the electrode portion is formed in the upper substrate, and then an electrode wire extending from the inside of the opening up to an upper surface of the upper substrate, which is connected to the electrode portion, is formed. In the upper surface of the upper substrate, the electrode wire and the external wire are wire-bonded to each other. Through the external wire and the electrode wire, the electrode portion and an external equipment are thereby connected to each other and a potential of the sensor portion and the like is drawn out and so on from the outside.
For the purpose of protecting the sensor portion from an external pressure, the thicknesses of the upper substrate and the lower substrate are required to be at least 300 to 400 μm or more.
As a method of forming the opening in the upper substrate having such a thickness, the following method is adopted as a background-art technique.
Specifically, a cavity is so formed in the upper substrate as not to penetrate it, up to a predetermined depth by sandblasting, and then the upper substrate and the lower substrate are so bonded to each other as to seal the sensor portion and the like. After that, the cavity is isotropically etched, to form the opening penetrating the upper substrate.
In order to form the opening in the upper substrate having a thickness of about 400 μm, for example, first, a cavity is formed by sandblasting up to a depth of about 300 μm. After that, the upper substrate having a thickness of about 100 μm which is present at a bottom of the cavity is completely removed by isotropic etching. In this isotropic etching, an upper surface of the upper substrate is entirely removed by about 100 μm.
Thus formed is the opening penetrating the upper substrate having a thickness of about 300 μm.
Further, there is a technique where the opening penetrating the upper substrate is formed only by sandblasting, not using isotropic etching.
The background-art acceleration sensor having the opening in the upper substrate is disclosed in Japanese Patent Application Laid Open Gazette No. 10-115635.
The case where the opening is formed in the upper substrate by performing sandblasting and then performing isotropic etching or the case where the opening is formed only by sandblasting has the following problem.
Since the opening is formed by sandblasting and isotropic etching, the upper substrate is also laterally removed and the diameter of the opening thereby increases (in the upper substrate having a thickness of about 300 μm, the opening has a diameter of about 50 to 200 μm at its bottom and a diameter of about 400 μm at its top).
In order to completely seal the cavity portion, it is required that the electrode portion should close the bottom of the opening without any clearance. Therefore, the electrode portion is required to have a size larger than the expected largest diameter of the opening (specifically, the size of the electrode portion=the expected largest diameter+a margin size).
Then, in the above background-art case, since the expected largest diameter is 400 μm, the size of the electrode portion is required to be 400 μm or more. Thus, since the expected largest diameter of the opening (specifically, the diameter at the top of the opening) is large in the above background-art case, the size of the electrode portion is set still larger.
As discussed above, the background-art case has to have the electrode portion of large size, and as a result, has a problem of making the acceleration sensor large in size on the whole.